Heliostat mechanical stop and method of finding heliostat home position

ABSTRACT

A drive assembly for a heliostat and a method of determining the drive&#39;s home position are described. The drive assembly comprises at least one mechanical hard stop for impeding the motion of an output gear of at least one gear transmission driven by a motor. Each hard stop may be built into the housing of a gear transmission or may be made integral with an output gear. The method for determining the drive assembly&#39;s home position comprises the steps of iteratively actuating an output gear towards, and then away from, a hard stop feature while monitoring the input torque of the driving motor. When the motor input torque is registered to be higher than a predetermined trip torque, the stepper motor is commanded to stop and the home position of the drive assembly is determined.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/881,373, filed on Sep. 23, 2013, the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to heliostats having reflectors configured to redirect sun light to a target or receiver. In particular, the invention relates to a mechanical stop of a gear train configured to limit the rotation of a heliostat drive assembly and the method of finding the home position of a heliostat using said mechanical stop.

In Concentrating Solar Power (CSP) plants, arrangements of heliostats reflect sunlight toward a receiver mounted atop a tower containing a working fluid. One type of receiver transfers incident radiant energy to the working fluid to produce high-pressure, high-temperature steam through the means of a heat exchanger or a phase change of the working fluid itself. The working fluid can be water, air, or a salt material heated to a molten state. The output steam can facilitate a variety of applications, such as electrical power generation, enhanced oil recovery, and desalination. Heliostats are generally mounted on the ground in an area facing or surrounding the receiver tower. Each heliostat has a reflector: a rigid reflective surface, such as a mirror, that tracks the sun through the actuation of a heliostat drive mechanism about at least one axis. Sun-tracking involves orienting the reflector throughout the day so as to optimally redirect sunlight from the sun toward the receiver and maintain the desired temperature of the working fluid.

One approach to constructing a heliostat field is to utilize a small amount of comparatively large heliostats (e.g., greater than between 50 and 150 m²). In such a power plant, having a fewer number of heliostats may necessitate the manufacture of very precise, and thus very expensive, components for the positioning of the reflective surfaces. Another approach, however, is to use a large amount of comparatively small heliostats (e.g., between 1 and 10 m²), such as with reflective surfaces that measure between 1 and 3 m on each side. Such an approach may be more efficient at redirecting sun light because there are more individually adjustable reflective surfaces. In addition, smaller heliostats may be cheaper to produce and easier to assemble, decreasing installation time and operations costs. However, a plant comprising more heliostats will necessarily require the same amount of additional drive assemblies, increasing the number of repeated steps during installation. Accordingly, there is a need for heliostat assemblies that are both economical to manufacture and efficient to install.

One problem with controlling the heliostats during plant operation is that sun-tracking must be precise, and the orientation of the reflective surface must be within a certain prescribed angular tolerance at all times. This orientation must be precise because accurate positioning of the reflectors is necessary to ensure that the requisite amount of light is targeted onto the receiver to maintain plant efficiency. A necessary condition for accurate heliostat positioning may be for the heliostat controller to always be apprised of the heliostat's current orientation. The orientation position may be determined by noting any deviation from the heliostat's zero position, or “home.” Defining a home position may also be useful for establishing a default orientation to which the heliostat may be reset in the event of a system restart or a reboot of a controller processor.

Additionally, wind and other environmental factors may apply loads to the reflector that may move it away from its preferred orientation at a given point in time while tracking the sun. Manufacturing tolerances between the components of the heliostat may also contribute to backlash, undesirable movement and non-linearity in the drive systems. These effects may undesirably result in a greater amount of variation between the expected and the actual reflector orientation. Such variation and external loading may cause a heliostat to attempt to traverse a range of motion beyond its design capabilities. For example, if the heliostat controller processor registers the initial orientation of the heliostat incorrectly, the heliostat may attempt to rotate about an axis farther than its mechanical constraints would normally allow. Consequently, inconsistency in the known heliostat orientation and anomalous motion may lead to excessive wear and tear to mechanical parts or even catastrophic failure. A conventional solution to this problem is to incorporate electronic limit switches that signal the motor controllers to cease operation when the switches are actuated. The heliostat drive assembly may be configured such that the limit switches are actuated when the heliostat has moved to a precise position, thereby establishing limits to the heliostat's range of motion. However, these limit switches may be unreliable or may malfunction, making heliostats vulnerable to failure and inaccurate positioning. Additionally, limit switches present an added cost to heliostat manufacturing in terms of both additional component parts and power consumption.

SUMMARY OF THE INVENTION

Improved heliostat drive assemblies are described herein, wherein the assemblies are configured to reduce the likelihood of mechanical component failure through the inclusion of mechanical stops in at least one gear train. A method of finding a heliostat home position is further described wherein the method comprises the steps of sensing when a heliostat has reached the limits imposed by a mechanical stop and then defining the heliostat's present orientation as a home position. The improved drive assemblies thereby reduce the risk of catastrophic failure resulting from drives actuating beyond their prescribed range of motion and improve the efficiency of their operation by facilitating a stable and findable home position.

A heliostat drive assembly may comprise a drive chassis, a drive shaft connected to the drive chassis, a reflector channel connected to the drive chassis, a first gear transmission comprising a first output gear, wherein the first gear transmission is configured to rotate said drive shaft via the first output gear about a first axis, a second gear transmission comprising a second output gear, wherein the second output gear transmission is configured to rotate said reflector channel via the second output gear about a second axis, a motor controller configured to actuate the first and second gear transmissions, a first stop feature configured to cease the motion of the first output gear of the first gear transmission, and a second stop feature configured to cease the motion of the output gear of the second gear transmission.

Each of the heliostat drive assemblies may have at least one axis of rotation for an attached reflector. For example, the heliostat drive assemblies may have two axes—an elevation axis and an azimuth axis, hence an “AZ-EL Heliostat.” The drive may have a prescribed range of motion about both axes that may be set according to its spatial dimensions and required functionality. This range of motion may be imposed by the locations of the mechanical hard stops relative to the traversable arc length of the outermost gears in the respective azimuth and elevation transmissions. When the drive controller is supplied a command to rotate the drive and an attached reflector about either the azimuth or elevation axes, the motors initiate the rotation of gears in said transmissions. In both the azimuth and elevation gear transmissions, there may be an output gear provided that works to rotate a shaft about which the drive or reflector rotates. When this output gear makes contact with a mechanical hard stop, the motor starts to output a higher torque in the face of resistance. When the motor torque becomes higher than a predetermined threshold torque, the motor may be stopped, preventing further rotation of the drive in the present direction about the given axis.

The reflector may be mounted to the drive via a reflector channel that may be actuated by the elevation gear transmission to rotate about the elevation axis. In an exemplary embodiment, the reflector maybe rotated in an arc no greater than 90 degrees from a home position about the elevation axis. The hard stop thus prevents the reflector from rotating so far about the elevation axis that it is brought into contact with the drive housing, which could cause damage to the mirror.

At least one of the azimuth and elevation transmissions may be configured such that there is a generally predetermined minimum biasing force on the one of the axes in a predetermined rotational direction. By having at least one of the axes biased in a predetermined rotational direction, the ability of wind loads and other external forces to cause undesirable motion, such as backlash, maybe significantly reduced and even eliminated. In an exemplary embodiment, the biasing force maybe provided by a spring, which may be tightened to a predetermined torsion. Excess loosening of the bias springs may result in insufficient biasing force for ameliorating the effects of backlash. Additionally, excess tightening of the bias springs may unduly impede the motion of the drive as it rotates opposite the direction of the bias force as well as result in dangerous mechanical failure if the spring is wound too tightly. The hard stop thus prevents the reflector from rotating about the azimuth or elevation axes in such a way as to cause the bias springs to loosen or tighten beyond acceptable torsion limits.

The hard stop features may also facilitate a method of finding the home position of the heliostat, wherein the method comprises the steps of:

-   (a) utilizing a motor controller having local memory storage to     actuate at least one motor to drive at least one gear transmission     having an output gear, wherein the output gear is configured to     rotate a drive assembly about an axis and wherein the output gear is     actuated in the direction of a hard stop feature; -   (b) measuring the input torque of the at least one motor with a     torque sensor while simultaneously driving the at least one gear     transmission with the motor; -   (c) measuring the step position of the at least one motor with the     motor controller while simultaneously driving the at least one gear     transmission with the motor; -   (d) commanding the motor to cease operation upon exhibiting a     predetermined condition; -   (e) recording the current motor step position to the memory of the     motor controller; and -   (f) defining the recorded motor position as the home stop position     in the motor controller memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a drive assembly for a heliostat, showing an azimuth gear housing with a cover and configured to rotate the drive assembly about an azimuth axis and an elevation gear housing with a cover and configured to rotate a reflector channel about an elevation axis;

FIG. 2 is a side view of the drive assembly of FIG. 1 showing the elevation gear housing with the cover removed and an elevation gear transmission, wherein the elevation gear transmission comprises an output elevation gear;

FIG. 3 is a perspective view of the elevation gear transmission removed from the elevation gear housing;

FIG. 4 is a perspective view of the output elevation gear of FIG. 2;

FIG. 5 is a perspective view of the drive assembly of FIG. 1 showing the elevation gear housing with the cover and elevation gear transmission removed;

FIG. 6 is a top view of the drive assembly of FIG. 1, showing the azimuth gear housing with the cover removed and an azimuth gear transmission comprising an output azimuth gear;

FIG. 7 is a perspective view of a first embodiment of the output azimuth gear of FIG. 5;

FIG. 8 is a perspective view of a second embodiment of the output azimuth gear of FIG. 5; and

FIG. 9 is a perspective view of the bottom of the azimuth gear housing cover and a hard stop feature made integral with the cover.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved AZ-EL heliostat assembly is described herein, with reference to FIGS. 1-9. The exemplary heliostat assembly is advantageously configured to provide for a mechanical stop that ceases movement outside of a prescribed range of motion, as described in greater detail herein.

An embodiment of a heliostat drive assembly 10, as illustrated in FIG. 1 comprises a chassis 1 and two gear transmissions (not shown), wherein each gear transmission may be enclosed in a respective gear housing. The chassis may comprise a single element or a plurality of elements bonded together or connected via fasteners. The gear housings may be made integral with the chassis and may comprise, for example, an azimuth gear housing 2 and an elevation gear housing 3. The azimuth gear housing 2 may enclose an azimuth gear transmission (not shown) for actuating the heliostat drive chassis about an azimuth axis. The azimuth axis may be defined as being orthogonal to the ground and passing through the center of an azimuth drive shaft 4. The elevation gear housing 3 may enclose an elevation gear transmission (not shown) for actuating a reflector channel 5 about an elevation axis. The elevation axis may be defined as orthogonal to the azimuth axis and co-linear with the center of an elevation output gear 8 (see FIG. 2). The elevation and azimuth gear transmissions may be driven by motors 7 that receive power from an internal or external power source. A reflector (not shown) may be mounted to the reflector channel 5 via fastener slots 6. The drive assembly may further comprise an azimuth torsion spring 9 that surrounds the azimuth drive shaft 4. The torsion spring may be configured to supply a biasing force on the azimuth output gear to prevent backlash in the azimuth gear transmission.

The elevation gear transmission, as illustrated in two perspectives in FIG. 2 and FIG. 3, may comprise a worm gear 11 driven by an input motor 7, wherein the worm gear interfaces with a first cluster spur gear. The first cluster spur gear may comprise primary and secondary gears 12 and 13 that are connected to each other or fixedly mounted to the same shaft, wherein the primary gear 12 has a larger diameter than the secondary gear 13, and wherein the primary gear is oriented between the secondary gear and the elevation gear housing cover (the cover has been removed in the illustrations of FIGS. 2 and 3). The secondary gear 13 of the first cluster spur gear may interface with a primary gear 14 of a second cluster spur gear, wherein the second cluster spur gear also may comprise primary and secondary gears 14 and 15 that are connected to each other or fixedly mounted to the same shaft, wherein the primary gear has a larger diameter than the secondary gear. The secondary gear 15 of the second cluster spur gear interfaces with the elevation output gear 8, wherein the elevation output gear works to rotate the reflector channel 5 about the elevation axis. The elevation gear transmission may be enclosed within elevation gear housing 3 and may be sealed by a housing cover (not shown) which may be fastened to the housing via mounting screw holes 16. An elevation torsion spring 17 may be connected to the elevation output gear 8. The torsion spring may induce a biasing force opposite a direction of rotation to reduce backlash in the elevation gear transmission.

The elevation output gear 8, as illustrated in FIG. 4, may comprise an arc segment of gear teeth 18, wherein the arc segment may be between 120 and 150 degrees. The center face of the output gear 8 may be outfitted with a plurality of bolt holes 19, through which fasteners may connect the output gear to the reflector channel 5, and a first spring retaining aperture 20, through which a first tip of an elevation torsion spring 17 (see FIG. 3) may be held. The output gear additionally comprises planar faces 21 at either end of the gear teeth arc segment.

The planar faces of the elevation output gear may make contact with elevation hard stop features 25, as illustrated in FIG. 5. The hard stop features serve to define the full range of motion of the output gear and cease any further movement of the heliostat in the elevation axis beyond the limits imposed thereupon. FIG. 5 displays the elevation gear housing 3 with all internal components removed. The drive chassis and elevation gear housing may be assembled from interconnected parts or formed from a single piece of material by, for example, die casting. The elevation gear housing 3 may comprise gear shaft bosses 22 and 23 which support the gear shafts upon which cluster spur gears 13 and 14 are mounted, respectively. The elevation gear housing may additionally comprise a second spring retaining aperture 24 through which a second tip of the elevation torsion spring 17 (see FIG. 3) may be held. The elevation gear housing 3 may additionally comprise elevation hard stop features 25, wherein the elevation heard stop features are planar surfaces formed into the side wall of the housing. The elevation hard stop features 25 cease movement of the elevation output gear 8 about the elevation axis when the planar faces 21 of the elevation output gear contact the planar surfaces of the hard stop feature.

FIG. 6 illustrates an open view of the azimuth gear housing 2 which houses the azimuth gear transmission. The azimuth gear housing and the drive chassis may be assembled from interconnected parts or formed from a single piece of material by, for example, die casting. The azimuth gear transmission may comprise an input motor 7 that receives power from an internal or external power source. The input motor 7 may drive a worm gear 26 mounted on a shaft, wherein the worm gear interfaces with a primary gear 27 of a first azimuth cluster spur gear, the first azimuth cluster spur gear additionally comprising a secondary gear 28 that is connected to the primary gear 28. The primary and secondary gears of the first azimuth cluster spur gear may be connected to each other or may be fixedly mounted to the same shaft. The primary gear 27 may have a larger diameter than the secondary gear 28, and the secondary gear 28 may be oriented between the primary gear 27 and the azimuth gear housing cover (the cover has been removed in the illustrations of FIG. 6). The secondary gear 28 of the first azimuth cluster spur gear may interface with a primary gear 29 of a second azimuth cluster spur gear, wherein the second azimuth cluster spur gear may additionally comprise a secondary gear 30. The primary and secondary gears of the second azimuth cluster spur gear may be connected to each other or may be fixedly mounted to the same shaft. The primary gear 29 may have a larger diameter than the secondary gear 30. The secondary gear 30 of the second azimuth cluster spur gear may interface with an azimuth output gear 31, which works to rotate the heliostat drive shaft 4 about the azimuth axis.

The azimuth output gear 31 may comprise a hollow center defining an annulus, an outer ring 32 and an inner ring 33, wherein the inner ring 33 has a smaller diameter than the outer ring 32 and a gap may be present between the inner ring and the outer ring. The azimuth output gear 31 may further comprise at least one protrusion 35 made integral with one or both of the inner and outer rings, wherein the protrusions are positioned within the gap between the outer ring 32 and the inner ring 33. The protrusions may be formed to exhibit shapes such as a triangle, trapezoid or rectangle, and may be formed as part of the gear itself or consist of additional parts that are attached to the gear. The protrusions may be formed from any material suitable for withstanding the force of impinging upon an azimuth hard stop feature 34, such as, but not limited to, steel or a steel alloy. The azimuth output gear 31 interfaces with the azimuth drive shaft 4 by insertion of the drive shaft into the hollow center of the gear. The azimuth drive shaft 4 may be held in place by, for example, a press fit, and may be rotatable by actuation of the azimuth output gear 31.

The azimuth gear housing 2 may be sealed by an azimuth gear housing cover (not shown) which may be fastened to the housing via mounting screw holes and comprises an azimuth hard stop feature 34. The azimuth hard stop feature 34 ceases movement of the azimuth output gear 31 about the azimuth axis when the planar face of the protrusions 35 come into contact with the azimuth hard stop feature 34. In this way, the position of the protrusions and the azimuth hard stop feature define the full range of motion of the azimuth output gear 31. In the present embodiment the azimuth output gear 31 may actuate through a maximum travel arc of 350 degrees from the home position.

In an alternative embodiment, the interface between the protrusion 35 and the azimuth hard stop feature 34 on the azimuth gear housing cover may incorporate a third part, an intermediate follower (not shown). The intermediate follower may comprise a second protrusion set along the arc of the azimuth output gear on either the inner or outer ring. The intermediate follower may be structured such that the azimuth output gear 31 may actuate through a cycle of motion greater than 360 degrees before contacting the azimuth hard stop feature 34.

A first embodiment of the azimuth output gear 31 is illustrated in FIG. 7. The azimuth output gear may be installed onto the end of the azimuth drive shaft 4 via, for example, a press fit. The azimuth output gear 31 comprises teeth that may interface with the secondary gear 30 of the second azimuth cluster spur gear (as shown in FIG. 6). The azimuth output gear 31 may have a rectangular protrusion 35 formed along the circumference of the outer ring 32 or the inner ring 33. In this way the protrusion may make contact with the planar face of the azimuth hard stop feature 34.

A second embodiment of the azimuth output gear 31 is illustrated in FIG. 8. In this second embodiment the azimuth output gear 31 may comprise two triangular protrusions 36, wherein the one protrusion is formed along the circumference of the outer ring 32 and the other along the inner ring 33. The two protrusions 36 may be aligned such that they are facing each other. In this way both protrusions may make contact with the planar faces of the azimuth hard stop feature 34.

The azimuth hard stop feature 34 may be formed on the underside of the azimuth gear housing cover 37, as illustrated in FIG. 9. The azimuth gear housing cover 37 may comprise gear shaft bosses 38 and 39 which support the gear shafts upon which the helical cluster spur gears 28 and 29 rotate, respectively. Additionally, the azimuth gear housing cover may further comprise an azimuth hard stop feature 34 having azimuth hard stop contact surfaces 40. The azimuth hard stop contact surfaces 40 are formed so as to make flush contact with the flat sides of the protrusions 35 of the azimuth output gear 31. The azimuth hard stop contact surfaces 40 and the protrusions 35 may comprise a variety of compatible shapes such that the sides of the protrusions can make flush contact with the hard contact surfaces. The azimuth hard stop feature 34 may further comprises ribs 41, which brace the hard stop surfaces 40 from excessive forces applied by the act of actuating the drive shaft to the limits of its range of motion. The azimuth hard stop contact surfaces 40 may be formed at an angle relative to the ribs 41. The angle of the azimuth hard stop contact surfaces 40 may be chosen such that the at least one protrusion 35 contacts the hard stop contact surface with as much surface area as possible.

The azimuth and elevation hard stops may help to mitigate catastrophic failures, which may result when the heliostat actuates through a range of motion that is greater than desired. Such failures may include potentially over-winding or unwinding the torsion bias springs, tangling or stretching power and communication cables, or damaging the reflector due to undesired contact with obstacles or the drive chassis. Unwinding the torsion springs reduce their effectiveness in preventing gear backlash, and over-winding of the torsion springs may produce unsafe conditions if the springs were to suddenly release. If the power and communication cables become excessively tangled or wound, they may become damaged and prevent operation of the heliostat. Therefore the azimuth and elevation hard stop features reduce the risk of performance degradation and prolong the lifespan of the device.

The present embodiment sets forth a compact heliostat drive, which may, for example, be sized to fit inside a square meter (without the attached reflector). Because the gear transmissions may have high gear ratios between 1600:1 to 2000:1, such as 1800:1, the drive motors may be low power units and still deliver the requisite torque, minimizing energy consumption of the units and helping to lower operation costs. Even at the present gear ratio, the small size of the heliostats reduces the need for a very robust mechanical hard stop, and so the present hard stop design may be easier to manufacture using reduced material quantities while still preventing physical breakage.

The presence of a hard stop feature at the outputs of both the azimuth and elevation gear transmissions provide numerous additional advantages, especially with regards to defining a “home stop” position. The heliostat input motors may be activated by motor controllers that may be housed within the drive chassis, the drive azimuth shaft or located external to the drive chassis. The motor controllers may have local memory storage capabilities and may be programmed to detect when the motor input torque is above a predetermined threshold value. This threshold value is called the “trip torque” and may be defined as a torque higher than the range of torque values exhibited by the motor during normal operation and lower than the “stall torque”, the torque at which the motor stops rotating. As an output gear interfaces with a hard stop feature in the azimuth or elevation gear transmissions, resistance from the planar surfaces of the hard stop introduce a region of gear torsion as the entire transmission “winds up”. This condition may be detected by the electronics of the motor controller board using software to monitor the input torque of the stepper motors using at least one torque sensor.

When the measured torque has reached or gone higher than the trip torque, the motor controller software may then command the motor to cease operation to prevent excessive torsion in the gears. Stopping the motor prior to it reaching a stalled condition maintains the commanded position of the drive without losing motor steps, providing for a more accurate determination of the heliostat's orientation. The trip torque level may be set low enough such that the output gear pressing against the surfaces of the hard stop feature will result in detection of the “winding up” condition, but not so low as to result in false positives from external forcing, such as light winds or other environmental externalities. Additionally, the motor step position at the hard stop location may be associated with a “home stop” position, providing a marker in the controller memory by which all future movements of the heliostat away from the home position may be measured. By forming the hard stop to be immutable and of a fixed position, the home position measurement may be repeatable and stable over many movement cycles.

To improve the accuracy and repeatability of the home stop determination, the drive motor controller software may further comprise a method for setting the home stop position after repeated movements of the output gears into the hard stop feature. The method of finding the hard stop may comprise a first step of actuating the heliostat drive to traverse its range of motion towards the home position. The motor controller software may then, upon detecting that the motor torque has become greater than the trip torque, which would be indicative of having contacted the hard stop and begun the “winding up” of the gear train, record the current motor step position and then command the motor to actuate the gears in the opposite direction, effectively backing away from the hard stop by a small number of steps. The output gear may then be moved back into the hard stop, repeating the steps of sensing the motor input torque levels with respect to the predetermined trip torque and then recording the “home” step position.

After a set number of iterations of backing away from the hard stop and contacting it again, the plurality of measured home step positions of the motor may be analyzed to determine a best guess for the true home position. This method of analysis may comprise, for example, the step of taking an arithmetic mean of all the home step positions measured during the homing attempt and setting the true home position to a calculated value. The analysis could also comprise a method of eliminating data samples obtained during the settling phase of the windup region. The method of analysis may further comprise a step of eliminating erroneous samples generated by premature sensing of an input torque higher than the trip torque or stop conditions exhibited by the drive prior to reaching the hard stop. For instance, if upon encountering a high motor input torque condition, the drive subsequently advances further than its current position by a predetermined number of steps, this may indicate that the first high motor input torque location was not at the true home position. The motor controller may then work to move the heliostat drive in the present axis to a newly calculated step position. This operation may make the home stop determination much more repeatable by filtering out false positives.

The threshold for detecting erroneous home positions may take into account acceptable variation between homing attempts. Incorrect home stop measurements may occur due to external applications of torque from environmental externalities or user error. The method of determining the home position may also include the step of recording the step position variance between home attempts and performing statistical analysis on the data set to ascertain if the home position is changing over time or has suddenly shifted. This analysis could prompt the motor controller software to recalculate the home position using the same or modified data set, initiate a new homing event, or flag the heliostat unit for inspection or repair.

Various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further it is intended that the scope of the present invention herein disclosed by way of examples should not be limited by the particular disclosed embodiments described above. 

We claim:
 1. A heliostat drive assembly comprising: a drive chassis; a drive shaft connected to the drive chassis; a reflector channel connected to the drive chassis; a first gear transmission comprising a first output gear, wherein the first gear transmission is configured to rotate said drive shaft via the first output gear about a first axis; a second gear transmission comprising a second output gear, wherein the second gear transmission is configured to rotate said reflector channel via the second output gear about a second axis; a motor controller configured to actuate the first and second gear transmissions; a first stop feature configured to cease the motion of the first output gear; and a second stop feature configured to cease the motion of the second output gear.
 2. The heliostat drive assembly of claim 2, wherein the first and second stop features comprise mechanical stops.
 3. The heliostat drive assembly of claim 2, wherein the first and second stops comprises flat surfaces.
 4. The heliostat drive assembly of claim 1, wherein the first stop is made integral with the drive chassis.
 5. The heliostat drive assembly of claim 2, wherein the first output gear comprises at least one protrusion.
 6. The heliostat drive assembly of claim 5, wherein the first output gear has an annulus shape and comprises an inner ring and an outer ring, wherein the outer ring has a larger diameter than the inner ring and a gap is present between the inner ring and the outer ring.
 7. The heliostat drive assembly of claim 6, wherein said protrusions are formed on the inner ring or on the outer ring.
 8. The heliostat drive assembly of claim 7, wherein said protrusions are collinear with the center of the first output gear.
 9. The heliostat drive assembly of claim 8, wherein said protrusions have planar faces.
 10. The heliostat drive assembly of claim 1, wherein the second stop is made integral with the drive chassis.
 11. The heliostat drive assembly of claim 1, wherein the second output gear comprises a tooth region having an arc segment between 120 and 150 degrees.
 12. The heliostat drive assembly of claim 2, wherein the second output gear further comprises at least one planar surface that makes flush contact with the second stop.
 13. The heliostat drive assembly of claim 1, wherein the motor controller has local memory storage.
 14. A method of determining the home position of a heliostat drive assembly, which comprises the steps of: (a) utilizing a motor controller having local memory storage to actuate at least one motor to drive at least one gear transmission having an output gear, wherein the output gear is configured to rotate a drive assembly about an axis and wherein the output gear is actuated in the direction of a hard stop feature; (b) measuring the input torque of the at least one motor with a torque sensor while simultaneously driving the at least one gear transmission with the motor; (c) measuring the step position of the at least one motor with the motor controller while simultaneously driving the at least one gear transmission with the motor; (d) commanding the motor to cease operation upon exhibiting a predetermined condition; (e) recording the current motor step position to the memory of the motor controller; and (f) defining the recorded motor position as the home stop position in the motor controller memory.
 15. The method of determining the home position of a heliostat drive assembly of claim 14, further comprising the steps of: (g) actuating the stepper motor to move the output gear away from the stop feature a predetermined number of steps; (h) repeating steps (a) through (f); and (i) repeating steps (g) and (h) a predetermined number of times.
 16. The method of determining the home position of a heliostat drive assembly of claim 14, wherein the predetermined condition of step (d) is the registering by the torque sensor of a higher input torque to the respective gear transmission than a predetermined trip torque. 